求助! 带有链接的图片为什么某些时候热区会偏移或消失？

代码如下：
再FF里，只要页面里有些信息（别的位置）刷新了，info.gif的链接热区就消失了或发生偏移，但另外两个图片的链接热区没有问题，（在IE里没此问题）哪位大侠知道是怎么回事或遇到过此类问题？谢谢

<table width="100%" height="85" border="0" cellpadding="0" cellspacing="0">
  <tr background="image/titlebar_center.gif">
    <td width="32" rowspan="3" background="image/titlebar_center.gif"><img src="image/titlebar_left.gif" width="22" height="85"></td>
    <td width="785" rowspan="3" background="image/titlebar_center.gif">&</td>
    <td height="29" colspan="3" background="image/titlebar_center.gif" class="tr">Current User: <TMPL_VAR NAME="username">
    </td>
  </tr>
  <tr background="image/titlebar_center.gif">
    <td width="66" height="30" align="right" valign="top" background="image/titlebar_center.gif"><a id="systemInfo" href="#"><img src="image/info.gif" width="38" height="32" alt="info" border="0" onClick="popwin_sysinfo = window.open('systemConfigure?rm=systeminfo', 'SystemInfo', 'toolbar=no,location=no,directories=no,status=no,menubar=no,scrollbars=no,resizable=yes,copyhistory=yes,width=700,height=450,left=150,top=100')"></a></td>
    <td width="63" align="right" valign="top" background="image/titlebar_center.gif"><a id="logout" href="sentinelConfigure?rm=logout" target="_parent"><img src="image/logout.gif" width="59" height="32" alt="layout" border="0"></a></td>
    <td width="63" valign="top" background="image/titlebar_center.gif"><a id="helpLink" href="sentinelConfigure?rm=help&action=general" onClick="popwin_help = popHelpWindow(this.href, 'sentinelhelp', '800', '600', 'yes', 'center'); return false" onFocus="this.blur()"><img src="image/btn_help.gif" width="39" height="32" alt="help" border="0"></a></td>
  </tr>
  <tr background="image/titlebar_center.gif">
    <td height="26" colspan="3" background="image/titlebar_center.gif">&</td>
  </tr>
</table>